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I've got a Dahon Jetstream XP (not that that matters for this question) that has had two flats today.

Rode it for the first time in a while. Tires are basically new maybe 300 miles on them give or take. Schwalbe Marathon Racer.

So I pump them up this morning. Commute to work. About 9:15 after sitting for two hours and not being touched the front tire just lets go. Hissssssssssss. I've got to be somewhere in a few minutes so I pop off the tire and change the tube in a hurry. Ride to that place. Ride home for lunch. 10 km or so. Then after lunch I ride maybe 15 km for work stuff and back to my office.

Hour later I have another appointment and the frigging tire is flat nearly. So I give it some blasts of air from the hand pump, go to the errand maybe 3 km and I'm nearly flat. Ride home instead of work because I figure I'll never get back and I've only got one spare tube left.

I get home and I chalk mark the tire to the rim. Remove the tire. Take the tube and pump it up and stuck it in a bucket and found the leak.

I go back and just inspect that general area of the tire and rim (based on where my chalk mark is) like nobodies business. Nothing.

I run my bare hand back and forth begging to get cut. Nothing.

Lint free towel looking for it to snag on something. Nothing.

Both tubes are same brand. I have not had a chance to inspect the first tube to see if the failure was in the same place. I'll do that tomorrow.

I patched the tube and will see if it holds tonight.

WTF? How can you get a pinhole in a tube with nothing? I even took a 10 power magnifying glass looking around the area of the rim and tire and I'm telling you there's nothing.

On the roads I ride, Marathon Racers become basically useless on the rear wheel after maybe 2000-2500kms of commuting or so (in front, they hold up fine). Dozens of small cuts in the outer surface, at the bottom of each cut is a small shrapnel working its way towards the inner tube. In my experience, a shrapnel can penetrate just far enough to cause a slow leak. Yet I cannot feel it in the inside of the tyre when there's no pressure and weight on the tyre. Look on the outer surface, twist the tyre all out of shape to find a cut and get it open enough to get the shrapnel out with small tweezers. And that's only temporary relief, since there are a bunch of them.

Sounds like your tyres are too fresh for this to happen, but I'd still check the chalk marked spot on the outer surface for cuts. With so little mileage, this is hopefully an isolated incident.

You probably have a small piece of wire stuck in the tire. Small enough that you cannot feel with your finger. When pressure is in the tire the wire will punch a hole and their is your flat. Take the tire off and turn it inside out and SLOWLY roll it and look for something protruding. It took me about 2 hours tofind one like that a couple of years ago.

I changed the tube yet again (different brand tube) and took a 30 minute ride. Just woke up and the tire is still holding air.

I think I had crappy tubes and having marked the tire to the rim I was able to very closely inspect the failure area on the tire. I rolled it as recommended and inspected with a 10 power magnifying glass and I see absolutely no indication of a puncture at all, inside or out of the tire.

I concur with Al1943 above. Check where the hole is. Sounds like though your familar with tires and flats thus it was probably on the tread side.

Thus Al1943's second statement about a faulty tube problem occuring within a batch out of the factory. I actually had that happen with two pairs of mail order Mich tubes I got for 1/2 the cost then the LBS in town...that's probably explains why they were 1/2 the cost! But these all started leaking on the tread side and it drove me nuts trying to find the problem. Finally I blew the tube up outside the tire and noticed a very, and I mean very, thin area about the size of dime (when blown up to about 2 inch in diameter) and that area opened up while I was pumping it.